A thin film transistor (TFT) for use in flat panel displays and the like has a structure wherein source and drain electrodes are disposed on one side of a semiconductor layer forming part of the thin film transistor and a gate electrode disposed on the other side of the semiconductor layer at a substantially central position relative to a channel.
Usually, a semiconductor layer of a thin film transistor having the above-described structure is formed by processing a semiconductor thin film formed on a substrate under precise control based on a thin film control process. Here, the semiconductor layer of the thin film transistor formed by the thin film control process calls for an excellent carrier mobility for a semiconductor device to be constructed. To this end, conventional thin film transistors preferentially employ inorganic semiconductors having excellent levels of carrier mobility such as silicon and germanium as semiconductor materials forming such semiconductor layers.
With recent prevalence of mobile information processing terminals such as PDA and the like, demands exist for realization of sheet-like or paper-like high-definition displays of low prices, microelectronic devices of low prices, and like devices. To meet these demands, realizations of inexpensive flexible electrodes and wiring and of flexible miniature thin film transistors of low prices are demanded. In forming such inexpensive flexible electrodes and thin film transistors or the like, it is required that conductor layers and semiconductor layers be imparted with flexibility by inexpensive means. For this reason, conductor and semiconductor materials are being sought which can replace the aforementioned inorganic semiconductors and which can impart resulting conductor and semiconductor layers with flexibility inexpensively.
In recent years, attention has been focused on electronic functional organic materials each comprising such an organic semiconductor compound as thiophene, pentacene or the like as a conductor material for forming a conductor layer to be used as an electrode, wiring or the like and as a semiconductor material for forming a semiconductor layer for use in such a semiconductor device as a thin film transistor, an organic electroluminescent device (organic EL) or the like.
Such an electronic functional organic material has a characteristic that it can be deposited at room temperature or at a temperature close to room temperature. Accordingly, use of such an electronic functional organic material makes it possible to form a conductor or semiconductor layer by a low-temperature process. For this reason, it is possible to form a conductor or semiconductor layer by the use of relatively inexpensive fabrication equipment adapted to the low-temperature process without the need to provide expensive fabrication equipment adapted to a high-temperature process that is needed when using an inorganic semiconductor. In brief, such a conductor or semiconductor layer can be formed inexpensively.
These electronic functional organic materials generally have flexibility. Accordingly, the use of such an electronic functional organic material for a conductor or semiconductor layer makes it possible to impart the layer with flexibility. For this reason, if a conductor or semiconductor layer comprising an electronic functional organic material is formed on a flexible plastic substrate or resin film, it becomes possible to fabricate an electrode, thin film transistor or the like having flexibility easily. Use of such a flexible electrode, flexible thin film transistor or the like enables a sheet-like or paper-like display or electronic device or the like to be constructed easily.
Examples of known thin film transistors of the type using an electronic functional organic material in a semiconductor layer include one having a semiconductor layer comprising oligothiophene which is an organic semiconductor compound. The semiconductor layer of this thin film transistor comprises an oligothiophene thin film formed by evaporating oligothiophene on a substrate or depositing oligothiophene on the substrate by coating the substrate with a solution of oligothiophene in an organic solvent. However, since the semiconductor layer of this known thin film transistor is formed by mere evaporation or deposition of oligothiophene without orientation or alignment, the thin film transistor has a far lower carrier mobility at channel than a thin film transistor using silicon or the like. The carrier mobility of such a thin film transistor having a semiconductor layer formed by mere evaporation or deposition of oligothiophene is about 0.001 cm2/Vs for example (see patent document 1 for example).
Thus, such a conductor or semiconductor layer comprising an electronic functional organic material has a problem that the carrier mobility thereof is considerably lower than that of a layer comprising an inorganic semiconductor. Specifically, the carrier mobility of single-crystalline silicon is about 103 cm2/Vs, that of polycrystalline silicon about 102 cm2/Vs, and that of amorphous silicon about 1 cm2/Vs, whereas that of a layer comprising such an electronic functional organic material as oligothiophene is about 0.001 to about 0.01 cm2/Vs.
The carrier mobility of the conductor or semiconductor layer comprising an electronic functional organic material is considerably lower than that of the layer comprising an inorganic semiconductor because the conductor or semiconductor layer formed by mere evaporation or deposition of the electronic functional organic material has electronic functional organic material molecules therein with their axes positioned disorderly, which impedes smooth charge transfer between plural electronic functional organic material molecules, thus lowering the electric conductivity of the layer. For this reason, it has been difficult for the prior art to use electronic functional organic materials in forming a conductor layer for use as an electrode, wiring or the like or in forming a semiconductor layer in a semiconductor device such as a thin film transistor, an organic electroluminescent device or the like, particularly, a semiconductor layer in a high-definition image display device, a high-speed LSI, or the like.
In attempt to overcome this difficulty, various studies have heretofore been made with a view to improving the carrier mobility of a conductor or semiconductor layer comprising an electronic functional organic material.
For example, a thin film transistor has been reported in which a crystalline low-molecular electronic functional organic material comprising pentacene or the like is used as an electronic functional organic material to form a semiconductor layer in which the low-molecular electronic functional organic material is oriented and aligned by an evaporation technique (see non-patent document 1 for example).
In this thin film transistor having the semiconductor layer formed by orienting and aligning the low-molecular electronic functional organic material, pentacene, which is an organic semiconductor material, is evaporated on a substrate to form the semiconductor layer of the thin film transistor. Here, pentacene is evaporated at a deposition rate of 1 Å/s on the substrate surface in which the temperature is room temperature (27° C.). This evaporation process causes pentacene molecules to be substantially aligned in the direction normal to the substrate thereby realizing a thin film phase having poor grain boundary. It has been reported that such a structure realized a thin film transistor having a semiconductor layer having a carrier mobility of about 0.6 cm2/Vs, which is a relatively high value among values of carrier mobility obtained from semiconductor layers formed by using electronic functional organic materials.
As another art, there has been disclosed a thin film transistor having a semiconductor layer comprising an organic semiconductor polymer in which a liquid crystalline substituent group is introduced at side chain, wherein the skeletal chains of the organic semiconductor polymer are aligned in a predetermined direction (see patent document 2 for example).
In this thin film transistor having the semiconductor layer formed by using the organic semiconductor polymer in which a liquid crystalline substituent group is introduced at side chain, the semiconductor layer comprises a liquid crystalline phase of a polythiophene derivative in which such a liquid crystalline substituent group as phenylcyclohexane (PCH)-type substituent is introduced at the 2- or 3-position of thiophene. It has been disclosed that the thin film transistor having a carrier mobility at channel of 6×10−5 cm2/Vs was obtained by orientation of the backbone chain axis of the thiophene polymer caused by orientation of the liquid crystalline substituent group in the liquid crystalline phase.
FIG. 19 is a sectional view schematically showing a sectional structure of the thin film transistor according to the aforementioned patent document 2.
As shown in FIG. 19, the thin film transistor 100 according to patent document 2 includes an insulating substrate 101 formed with a gate electrode 103, and an organic semiconductor film 106 formed over the insulating substrate 101 with a gate insulator 102 intervening therebetween. Further, between the insulating substrate 101 and the organic semiconductor film 106 are formed a source electrode 104 and a drain electrode 105 in such a manner as to connect to the organic semiconductor film 106 directly. The organic semiconductor film 106 is formed by a process including: polymerizing a PCH-type liquid crystal compound (PCH504) and thiophene by a catalytic polymerization method; dissolving the resulting polymer in a chloroform solvent to exhibit a liquid crystalline phase; and coating the upper sides of the gate insulator 102, source electrode 104 and drain electrode 105 with the resulting solution to a film thickness of 1 μm by a casting method. The organic semiconductor film 106 extending over the source and drain electrodes 104 and 105 is subjected to a treatment for orientation control. In the organic semiconductor film 106, the liquid crystalline substituent group introduced in the organic semiconductor polymer forming the organic semiconductor film 106 can be oriented parallel with a rubbing direction. Since the organic semiconductor film 106 formed is thin, the skeletal chains of the organic semiconductor polymer are aligned in a fixed direction relative to the liquid crystalline substituent group, or side chain. Stated otherwise, in the thin film transistor 100 the direction in which the skeletal chains of the organic semiconductor polymer are aligned is controlled by controlling the alignment direction of the liquid crystalline substituent groups by an orientation treatment.
In recent years, attention has also been focused on nanotube (NT) having a nanostructure, particularly, carbon nanotube (CNT), which is an inorganic compound formed from carbon (C) as a semiconductor material for forming a conductor or semiconductor layer.
In these days, many studies are being made of nanotube (NT) and carbon nanotube (CNT) since they exhibit very good electric conductivity and high mechanical strength and are very stable chemically and thermally.
A carbon nanotube has a very small diameter on a nanometer order and a length on a micrometer order and hence has a very high aspect ratio. Therefore, carbon nanotube is extremely close to an ideal one-dimensional system. Such carbon nanotubes include a metallic carbon nanotube having a high electric conductivity and a semiconductive carbon nanotube having a band gap in inverse proportion to the diameter thereof, which are produced in accordance with the diameter and the helical degree which depend upon the symmetry of the molecular structure thereof. Usually, carbon nanotube is synthesized as a carbon nanotube mixture comprising the metallic carbon nanotube and the semiconductive carbon nanotube in proportions of about 1:2 for example. Since the metallic carbon nanotube has a high electric conductivity, it is possible to use the metallic carbon nanotube as a favorable wiring material or as a conductive member for use in devices of microstructure. In using carbon nanotube in a semiconductor layer of a thin film transistor, it is required that the semiconductive carbon nanotube be used. Such a thin film transistor having a semiconductor layer formed by using the semiconductive carbon nanotube can obtain a very high carrier mobility at channel of 1000 to 1500 cm2/Vs.
A report has been made of a nanotube-type thin film transistor having a semiconductor layer comprising carbon nanotube, wherein the semiconductor layer is formed to a thickness of about 1.4 nm by dispersing carbon nanotubes each having a diameter of about 1.4 nm at an appropriate dispersion density (see non-patent document 2 for example).
FIG. 18 is a sectional view schematically showing the construction of a thin film transistor having a semiconductor layer comprising carbon nanotubes.
According to non-patent document 2, a thin film transistor 200 as shown in FIG. 18 includes a 150 nm-thick gate insulator 202 comprising thermally oxidized silicon formed on an upper portion of a p+ silicon substrate 201 which serves also as a gate electrode, and a 1.4 nm-thick semiconductor layer 203 formed by dispersing semiconductor-type carbon nanotubes of 1.4 nm diameter on the gate insulator 202 at an appropriate dispersion density. Also, a metal such as titanium (Ti) or cobalt (Co) is deposited on the surface of the semiconductor layer 203 by evaporation and, further, source electrode 204 and drain electrode 205 each comprising titanium carbide or cobalt are formed on upper sides of contact sections 206 and 207 in contact with the carbon nanotubes. The thin film transistor 200 is thus constructed. Such a construction yields a nanotube-type thin film transistor having a sufficiently high carrier mobility at channel and favorable electrical characteristics.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2000-029403
Patent Document 2: Japanese Patent Laid-Open Publication No. HEI 09-083040
Non-patent Document 1: C.D. Dimitrakopoulos and another one, IBM J. RES. & DEV. VOL. 45 NO. 1 JAN. 2001 pp 19
Non-patent Document 2: PhaedonAvouris, Chem. phys. 281, pp. 429-445 (2002), FIG. 6, “Carbon nanotube electronics”